Fail-safe assembly for gas-lift production systems

ABSTRACT

A fail-safe system is provided for use with gas-lift production of oil from a well, including a novel blocking valve actuable to interrupt gas injection in response to an excessive pressure differential in the injection line. In particular, the valve stem in the blocking valve is spring-loaded open and is also arranged to be urged open by the low pressure side of a differential upstream in the flow line. A pneumatic piston is arranged to urge the stem into the valve seat by the high pressure side of the differential, but the spring is selected to exceed the normally expected differential between the low and high pressures.

United States Patent 1,478,320 12/1923 Baxter 2,027,762

Everett D. McMurry Houston, Tex.

July 22, 1969 Apr. 20, 1971 McMurry Oil Tools, Inc. Houston, Tex.

lnventor Appl. No. Filed Patented Assignee References Cited UNITEDSTATES PATENTS 1/1936 Becker 137/498 2,418,743 4/1947 Baker l37/498X2,674,261 4/1954 Wilson 137/498 2,834,371 5/1958 Liljestrand 137/46 1 XABSTRACT: A fail-safe system is provided for use with gaslift productionof oil from a well, including a novel blocking valve actuable tointerrupt gas injection in response to an excessive pressuredifferential in the injection line. In particular, the valve stem in theblocking valve is springloaded open and is also arranged to be urgedopen by the low pressure side of a differential upstream in the flowline. A pneumatic piston is arranged to urge the stem into the valveseat by the high pressure side of the differential, but the spring isselected to exceed the normally expected differential between the lowand high pressures.

PATENTEUAPRZUIQH 3575204 SHEET 1 UF 2 FIG. I

Everett D. McMurry INVENTOR A TTORNEYS ATENTEDA 3,575,204

' sum 2 UF 2 I, i I

i 74 8 1 Hi FIG. 3

E ver ett D. Mc Murry /N VE N TOR BY flruwgcl, Mu,

WXIUMM ATTORNEYS FAIL-SAFE EMIBIJY FOR GAS-LIFT PRODUCTION SYSTEMSBACKGROUND OF INVENTION This invention relates to methods and apparatusfor fluid flow control, and more particularly relates to methods andapparatus for interrupting the flow of injection gas used for gas-liftoperations in oil wells and the like in cases of increases in thedelivery volume of such injection gas above a preselected maximumvolume. The invention is specifically directed to a novel blocking valveassembly especially suited to gas flow control.

There are many electricallyactuated devices for interrupting fluid flowupon the occurrence of certain predetermined circumstances. However,there are also many situations where it is either impractical orundesirable to utilize such electrically-actuated equipment.

For example, natural gas is often injected into oil wells for thepurpose of lifting the oil to the surface. In most such cases, however,it is necessary to utilize such injection gas only at pressures within arelatively narrow range. For example, if the injection pressure fallsbelow a certain minimum, the gas pressure will not open the gas-liftvalve and enter the tubing or producing conduit, and oil production willstop. If the differential pressure is too great between the injectionconduit (casing) and the production conduit (tubing), however, more gaswill be injected than is actually required to lift the oil through theproduction conduit to the surface.

Gas-lift operations are generally unattended, and are merely checked byinspection personnel at occasional intervals. Thus, abnormal pressurefluctuations in either the injection conduit or the production conduitcan produce a condition which may continue unabated or uncorrected foran extended period of time.

If a leak occurs in the tubing (production conduit) in the well, or ifone of the gas-lift valves fails, this will cause an abnormal pressuredrop to occur in the casing (injection conduit) in the well which, inturn, will create an abnormal increase in the pressure differentialbetween the casing and the gas injection line leading to the wellhead.Accordingly, the injection rate of gas into the casing will riseproportionately which, in turn, will proportionately increase the flowof injection gas into the tubing and out of the well without increasingthe flow of oil to any significant extent.

Heretofore, this did not create a serious problem for most welloperators, since the field price of gas has always been quite low inrelation to the field price of oil. The present day value of field gasin now many times greater than its former price, however. Furthermore,it is conventional to operate most wells with only periodic inspection.Accordingly, an abnormal increase in the pressure differential betweenthe casing and the gas injection line may cause the loss of hundreds ofthousands of cubic feet of injection gas during only a relatively shorttime interval, and may cause an otherwise profitable oil well to operateat a substantial loss.

Accordingly, there is now a substantial need for blocking means whichwill automatically interrupt the flow of injection gas into the wellwhenever an abnormal increase occurs in the pressure differentialbetween the injection gasline and the casing or other injection conduitin the well. Electrically operated equipment which is responsive to apressure difierential has long been available. However, most oil wellsare remotely located relative to conventional sources of electricalpower. Furthermore, it is undesirable for reasons of tire and explosionsto utilize electrically operated equipment in the immediate vicinity ofpipelines or other equipment handling oil or gas under high pressures.Thus, the particular need is for mechanically actuated blockingequipment for automatically interrupting gas flow into the well in sucha contingency.

SUMMARY OF INVENTION In a larger sense, the preferred apparatusembodiment of the present invention includes a closure means operable incombination with means to create a pressure differential and responsiveto any abnormal increase in such pressure drop or differential to closeand block fluid flow in the flow line sought to be controlled. As willhereinafter be explained in greater detail, the blocking valve assemblyis therefore preferably installed adjacent a component such as anorifice, which provides a normal pressure drop at a selected location inthe gas injection line leading to the wellhead. The pressure drop may beestablished solely and specifically for the purpose of actuating theblocking valve assembly, or the valve assembly may be located adjacentsome component (such as a needle valve), which creates a pressure dropin the line but which is installed primarily for some other reason.

As will also hereinafter be apparent, the subject blocking valveassembly includes a closure assembly, which is springloaded so as to benormally open. A pressure-operated driving means, such as a piston, isarranged and connected with the upstream side of the orifice or othercomponent producing the subject pressure drop, whereby the piston tendsto be driven against the closing member or valve stem to drive it intoshutoff engagement with an appropriate valve seat. The pressuredownstream of the orifice or like component, however, is routed againstthe valve stem to urge it out of engagement with the valve seat.

The compression force in the valve spring is normally greater than thepressure differential across the injection orifice. Accordingly, thevalve spring will keep the blocking valve assembly in its normally-opencondition provided the pressure differential does not exceed theopposing force of the valve spring.

An abnormal increase in pressure upstream of the orifice will, in theabsence of a corresponding increase in downstream pressure, provide anincrease in the pressure differential across the orifice or othercomponent used to establish the subject pressure drop in the line.Alternatively, (and more frequently in the case of gas-lift operation) adecrease in pressure downstream will produce the same increase inpressure drop. In either case, the increase in pressure differentialwill operate to shut the valve and interrupt line flow the instant itexceeds the strength of the valve spring. The valve spring may, ofcourse, be selected to establish the maximum pressure differential to betolerated.

These and other features and advantages will become apparent from thefollowing detailed description, wherein reference is made to the FIGS.in the accompanying drawings.

IN THE DRAWINGS FIG. it is a simplified pictorial representation offluid flow line incorporating an orifice for establishing a drop in linepressure, and further incorporating a suitable blocking valve assemblywith provision for sensing and responding to such drop or difierentialin line pressure.

FIG. 2 is a simplified functional representation of a typical oil wellinterconnected with a gas-injection line for gas-lift purposes, whereinthe aforementioned blocking valve assembly is operably interconnectedwith a flow rate stabilizer.

FIG. 3 is a more detailed representation, partly in cross section, ofthe blocking valve assembly depicted in FIGS. 1 and 2.

DETAILED DESCRIPTION Referring now to FIG. 1, there may be seen an idealembodiment of the present invention, wherein means is provided in a flowline for establishing a pressure drop at a selected line location whichis relatively minimal under normal line pressure conditions, and furtherincluding a flow interruption means sensible to and actuable by anincrease in such pressure differential above a preselected maximumdifferential. In particular, a conventional orifice 11 may be installedin a pipe or other flow line 7 to create the desired pressuredifferential, and a suitable blocking valve assembly 8 is preferablyinterconnected in the flow line 7 immediately downstream of the orifice11.

As may be seen in FIG. 1, a pressure line 9 is interconnected at one endto the blocking valve assembly 8, and at the other end to the flow line7 at a location upstream of the orifice 11, whereby upstream linepressure is continually applied to a suitable portion of the blockingvalve assembly 8. The valve assembly 8 is also inherently subject to theline pressure downstream of the orifice 11, and thus the valve assembly8 is always subject to whatever line pressure differential exists in theflow line 7 across the orifice 11. As previously stated, the blockingvalve assembly 8 is adapted to be normally open to pass fluid fiowthrough the flow line 7, and to remain open as long as the pressure dropor differential across the orifice 11 does not exceed a preselectedmaximum.

Referring now to FIG. 2, there may be seen a representation of analternative embodiment of the present invention incorporated in thegas-injection line of an oil well operated by a gas-lift system. Inparticular, there may be seen a simplified representation of an oil wellcontaining a production tubing 3 centrally located in a conventionalwell casing 2. A functional representation of a well head assembly 4 issuggested, including suitable gate valves 5 and 6.

In a system of the character depicted, gas is injected into the annulusof the casing 2 from the aforementioned flow line 7. One or moregas-lift valves 3A may be mounted conventionally along the tubing 3, andarranged to open to admit gas into the tubing 3 whenever the pressure inthe casing 2 exceeds a preselected magnitude. The pressure in the tubing3 is lower than the pressure in the casing 2, and thus the gas tends toexpand upon entering the tubing 3 to lift any liquids therein to thesurface of the earth.

Accordingly, a blocking valve assembly 8 is desirably incorporated inthe flow line 7 carrying injection gas into the casing 2, for thepurpose of closing in response to an excessive pressure differential. Asmay be seen in FIG. 2, a suitable flow rate controller 10, of the typedepicted in the copending US. Pat. application Ser. No. 794,189, whichwas filed .Ian. 27, 1969, by Everett D. McMurry and Bolling A,Abercrombie, may be included for the purpose of maintaining apreselected injection rate notwithstanding fluctuations in line pressureeither upstream or downstream of the controller 10. A pressure drop ofmeasurable magnitude will occur across a component such as the flow ratecontroller 10. Accordingly, the blocking valve assembly 8 is preferablylocated downstream of the flow rate controller 10, and the pressure line9 is preferably connected at a location upstream of the controller 10,as indicated in FIG. 2.

Referring now to FIG. 3, there may be seen a more detailedrepresentation of the blocking valve assembly 8 depicted in FIGS. 1 and2. In particular, a valve body 12 is provided, which has a generallyT-shaped configuration, and which includes threaded intake and outletports 24 and 25 for interconnection into the flow line 7. A generallycylindrical and hollow barrel member 13 may be seen to be threadedlyinterconnected with the valve body 12, with a suitable O-ring 28included therebetween to insure a gastight connection. An annular valveseat member 22 may be seen to be slidably inserted in the valve body 12,and provided with a shoulder portion for engaging the internal portionsof the valve body 12, whereby it is prevented from being dislocatedduring closure of the valve assembly 8. An O-ring 26 may be provided asindicated, to insure a gastight fit between the seat member 22 and theadjacent surfaces of the valve body 12 and to provide for gas flow onlythrough the orifice in the valve seat member 22.

A suitable ringlike member, hereinafter called a seat cage 23, is alsopositioned in the valve body 12 to lock the valve seat member 22 intolocation when the barrel 13 is properly interconnected with the valvebody 12. In addition, the seat cage 23 includes a suitable aperture toprovide communication between the intake port 24 and the valve seatmember 22. As may be further seen, the upper end of the barrel 13 isclosed or stoppered by a suitable barrel plug 15 threadedly connectedtherein, the barrel plug 15 having a threaded input aperture forreceiving the threaded downstream end of the pressure line 9 depicted inFIGS. 1 and 2. The barrel plug 15 is also provided with an outletaperture located opposite of the pressure line 9, and interconnectedwith a suitable reset valve 17 by means of a suitably threaded nipple16, or the like.

The shutoff components of the blocking valve assembly 8, may be seen toinclude a suitable valve stem 20, having a shutoff end portion at oneend formed for shutoff engagement with the confronting portions of theseat member 22, and threadably connected at its opposite upper end to astem holder 19. As may also be seen in FIG. 3, the stem holder 19 isprovided at its upper end with an enlarged shoulder or springretainingportion 19A. The barrel 13 is provided with a cylindrical internalconfiguration, narrowed along its lower end, to slidably support thestem 20, whereby the stem 20 is guided properly to and from the valveseat member 22 along a vertical axis of travel. A suitable O-ring 27 mayalso be included for the purpose of preventing gas leakage along theabutting surfaces of the valve stem 20 and barrel 13. A suitable valvespring 21 may be disposed in the larger upper cylindrical internalportion of the barrel member 13, for compression between the lower orundersurface of the springretaining shoulder portion 19A of the valvestem holder 19, and the upper surface of the internal shoulder definedby the smaller internal portion of the barrel 13. Thus, the compressedvalve spring 21 will tend to expand upwardly to hold the valve stem 20out of blocking engagement with the valve seat member 22.

A suitable power piston 18 may be slidably disposed within the upper endof the barrel plug 15, and may be provided with a narrower uppercylindrical end portion, and a larger lower cylindrical end portion,whereby pressure in the barrel plug 15 about the narrower portion of thepiston 18 tends to act against the larger end portion. The larger lowerend portion of the power piston 18 is slidably disposed between theadjacent inside surfaces of the barrel plug 15 to function as agasoperated piston and a suitable O-ring 29 may be provided to preventgas seepage therebetween. The narrower upper end portion of the powerpiston 18 is also slidably disposed between adjacent inside surfaces ofthe barrel plug 15, for the purpose of trapping pressure within thebarrel plug 15, and a suitable O-ring 30 may also be provided to obviategas seepage therebetween.

As hereinbefore stated, the object of the blocking valve assembly 8 isto close automatically in response to an abnormal pressure differentialacross a selected location in the pipeline 7, to prevent waste ofinjection gas because of either an abnormal increase in upstreampressure, or an unusual or abnormal decrease in downstream pressure.Pressure entering through the intake port 24 will be lower than the gaspressure entering the barrel plug 15 through the pressure line 9, forthe reasons hereinbefore given. Thus, gas pressure in the barrel plug 15tends to act against the larger lower portion of the power piston 18,and to urge the power piston 18 down into abutting engagement with thespring-retaining portion of the stem holder 19, to thereby drive thevalve stem 20 into shutoff engagement with the valve seat member 22.However, the force being applied to drive the power piston 18 downwardis only equal to the pressure differential between the upstream pressurein the pressure line 9 and barrel plug 15, and the downstream pressurein the intake port 24 and across the valve seat member 22. The opposingforce of the compressed valve spring 21 is preselected to be greatenough to overcome or exceed the pressure differential, and thus thevalve spring 21 will normally maintain the valve stem 20 in a raisedposition notwithstanding the upstream pressure being applied to thepower piston 18. If an abnormal increase in upstream pressure occurs,however, the differential between the pressure across the power piston18 and the pressure across the valve stem will be correspondinglyincreased to exceed the opposing force of the compressed valve spring21. Alternatively, if the downstream pressure decreases abnormally, thesame increase in pressure differential will result. in either case, thisincrease in pressure difi'erential will tend to urge the power piston l8downward to, in turn, urge the valve stem 20 into shutoff engagementwith the valve seat member 22.

It will be noted that if either of these abnormal pressure conditions isremoved, and if the pressure differential returns to normal, thecompressed valve spring 21 will again draw the valve stem 20 out ofshutoff engagement with the valve seat member 22. It sometimes occurs,however, that the aforementioned abnormal pressure conditions cannot beconveniently corrected. If it is nevertheless desired to continue toinject gas into the casing 2, the reset valve 17 may be opened to bypassor release pressure from the power piston 118, whereupon the powerpiston 18 will be urged upwardly in the barrel plug l5 by the valvespring 21, and the valve stem 2'0 will be drawn upward.

it will be noted that the lower portion of the barrel plug l5 isprovided with an enlarged cylindrical cavity for slidably receiving thespring-retaining portion 19A of the stem holder 119 in order to maintainthe stem 20 in coaxial alignment with the valve seat body 22. Since theO-rings 27 and 29 tend to create a gastight compartment within thebarrel 13, one or more notches or the like may be desirably provided inthe rim of the retaining portion 19A of the stern holder 19 tofacilitate movement of the stem 20. In addition, it may be desirable toprovide a vent R4 in the barrel 13 as depicted in FIG. 3 in order toprovide a fixed pressure within the barrel l3 and against thespring-retaining portion 19A of the stem holder 19 irrespective of theposition of the stem holder 19 within the barrel 13.

It will be apparent from the foregoing that various modifications andvariations may be made in the structures and procedures described hereinwithout substantial departure from the essential concept of the presentinvention. Accordingly, it should be clearly understood that the formsof the invention described herein and depicted in the accompanyingdrawings, are exemplary only and are not intended as limitations on thescope of the present invention.

lclaim: l. A flow control system for interconnection with a flow line tocontrol the supply of injection gas into an oil well or the like, saidsystem comprising:

differential means for establishing a pressure differential at a firstlocation along said flow line, and

normally open blocking means at a second location along said flow linefor interrupting said supply of injected gas in response to an increasein said pressure differential at said first location above a preselecteddifferential.

2. The control system described in claim 1, wherein said blocking meanscomprises:

a valve seat member disposed adjacent said second location,

a shutoff member urged away from said seat member by pressure in saidflow line on one side of said differential means, driving means forurging said shutoff member into engagement with said seat member bypressure in said flow line on the other side of said differential means,and

spring loading means urging said shutoff member away from said seatmember by a preselected force not greater than said preselecteddifferential.

3. The control system described in claim 2, wherein said second locationis downstream of said first location and said pressure on said otherside of said differential means is greater than said pressure betweensaid first and second locations.

6. The control system described in claim 3, wherein said differentialmeans includes:

throttling means located in said flow line upstream of said valve seatmember and shutoff member and providing a flow restriction at said firstlocation.

5. The control system described in claim 4, wherein said throttlingmeans is adapted to stabilize the flow rate of fluid flow between saidfirst and second locations in said flow line.

6. A blocking valve assembly for interrupting fluid flow in response toan excessive pressure differential, said assembly comprising:

a hollow body member for interconnection in a flow line and having anintake port and an outlet port,

an annular valve seat member interconnected in said body member betweensaid intake and outlet ports and across the flow path of said fluid,

means for establishing a pressure differential at a location upstream ofsaid seat member,

a normally open valve shutoff member for engaging said seat member whenthe pressure differential at said location is greater than a preselecteddifferential,

a cylindrical valve stem having one end connected to said shutoffmember,

guiding means slidably and gas-tightly supporting said valve stem,

spring means having a preselected compression strength andinterconnected with said valve stem to urge said shutoff member out ofshutoff engagement with said valve seat member, and

driving means adjacent the opposite end of said valve stem for urgingsaid stem against said spring means in response to the pressure upstreamof said location.

7. The blocking valve assembly described in claim 6,

wherein said driving means includes:

cylinder means having a pressure chamber communicating with saidupstream pressure, I

a piston member slidably disposed in said cylinder means and arranged tobe urged into abutting engagement with said opposite end of said valvestem in response to said upstream pressure in said pressure chamber.

8. A blocking valve assembly responsive to an excessive pressuredifferential for interrupting fluid flow, comprising:

a body member adapted for interconnection in a flow line,

an annular valve seat member mounted in said body member,

a hollow barrel member interconnected with said body member and having acylindrical passageway in one end adjacent to and axially aligned withsaid seat member,

a cylindrical valve stem having a shutoff surface at one end confrontingsaid seat member and slidably disposed in said cylindrical passageway,

a spring member having a preselected compression strength and disposedin said barrel member for urging said stem from said valve seat member,

pressure cylinder means interconnected to stopper the other end of saidbarrel member and having a cylindrical borehole opening at one endtoward said valve stem and a gas port communicating with saidcylindrical borehole adjacent its other end,

means for establishing a pressure differential at a location upstream ofsaid valve seat member and for communicating the upstream pressure atsaid location to said gas port, and

piston means slidably disposed in said cylindrical borehole and adaptedto be urged against said valve stem in response to gas pressure receivedfrom said gas port for urging said valve stem into shutoff engagementwith said valve seat member.

9. The valve assembly described in claim 8, wherein said valve stem isfurther provided with a spring-engaging portion, and

wherein said spring member comprises a helical spring disposed coaxiallyabout said valve stem and interconnected with said spring-engagingportion thereof.

10. The valve assembly described in claim 9, wherein said barrel memberis threadedly interconnected generally transversely of said body memberand is further provided with an internal shoulder portion for supportingand maintaining said helical spring in a compressed condition, and

wherein said spring-engaging portion of said valve stem is a flangelikeportion located at the opposite end of said valve stem from said shutoffsurface and adapted for abutting engagement.

11. A blocking valve assembly responsive to an excessive pressuredifferential for interrupting fluid flow, comprising:

a body member adapted for interconnection in a flow line,

an annular valve seat member mounted in said body member,

a hollow barrel member interconnected with said body member and having acylindrical passageway in one end adjacent to and axially aligned withsaid seat member,

a cylindrical valve stem having a shutoff surface at one end confrontingsaid seat member and slidably disposed in said cylindrical passageway tobe urged from said seat member by gas pressure in said body member.

a spring member having a preselected compression spring and disposed insaid barrel member for urging said stem from said valve seat member.

pressure cylinder means interconnected to stopper the other end of saidbarrel member and having a cylindrical borehole opening at one endtoward said valve stem and at least two gas ports each communicatingwith said cylindrical borehole adjacent its other end,

piston means slidably disposed in said cylindrical borehole and adaptedto be urged against said valve stem in response to gas pressure receivedfrom one of said gas ports for urging said valve stem into shutoffengagement with said valve seat member and closure means stoppen'ng theother of said gas ports and openable for releasing gas pressure fromsaid piston means.

POMS I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.qyw sgoq Dated Avril 20. 1970 lnvent fl i Everett D. McMurrv It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

line 67, delete "loading".

Signed and sealed this 10th day of August 1971.

(SEAL) Attest:

WILLIAM E. SCHUYLER, J'R.

EDWARD M.FLETCHER,JR.

Commissioner of Patents Attesting Officer

1. A flow control system for interconnection with a flow line to controlthe supply of injection gas into an oil well or the like, said systemcomprising: differential means for establishing a pressure differentialat a first location along said flow line, and normally open blockingmeans at a second location along said flow line for interrupting saidsupply of injected gas in response to an increase in said pressuredifferential at said first location above a preselected differential. 2.The control system described in claim 1, wherein said blocking meanscomprises: a valve seat member disposed adjacent said second location, ashutoff member urged away from said seat member by pressure in said flowline on one side of said differential means, driving means for urgingsaid shutoff member into engagement with said seat member by pressure insaid flow line on the other side of said differential means, and springloading means urging said shutoff member away from said seat member by apreselected force not greater than said preselected differential.
 3. Thecontrol system described in claim 2, wherein said second location isdownstream of said first location and said pressure on said other sideof said differential means is greater than said pressure between saidfirst and second locations.
 4. The control system described in claim 3,wherein said differential means includes: throttling means located insaid flow line upstream of said valve seat member and shutoff member andproviding a flow restriction at said first location.
 5. The controlsystem described in claim 4, wherein said throttling means is adapted tostabilize the flow rate of fluid flow between said first and secondlocations in said flow line.
 6. A blocking valve assembly forinterrupting fluid flow in response to an excessive pressuredifferential, said assembly comprising: a hollow body member forinterconnection in a flow line and having an intake port and an outletport, an annular valve seat member interconnected in said body memberbetween said intake and outlet ports and across the flow path of saidfluid, means for establishing a pressure differential at a locationupstream of said seat member, a normally open valve shutoff member forengaging said seat member when the pressure differential at saidlocation is greater than a preselected differential, a cylindrical valvestem having one end connected to said shutoff member, guiding meansslidably and gas-tightly supporting said valve stem, spring means havinga preselected compression strength and interconnected with said valvestem to urge said shutoff member out of shutoff engagement with saidvalve seat member, and driving means adjacent the opposite end of saidvalve stem for urging said stem against said spring means in response tothe pressure upstream of said location.
 7. The blocking valve assemblydescribed in claim 6, wherein said driving means includes: cylindermeans having a pressure chamber communicating with said upstreampressure, a piston member slidably disposed in said cylinder means andarranged to be urged into abutting engagement with said opposite end ofsaid valve stem in response to said upstream pressure in said pressurechamber.
 8. A blocking valve assembly responsive to an excessivepressure differential for interrupting fluid flow, comprising: a bodymember adapted for interconnection in a flow line, an annular valve seatmember mounted in said body member, a hollow barrel memberinterconnected with said body member and having a cylindrical passagewayin one end adjacent to and axially aligned with said seat member, acylindrical valve stem having a shutoff surface at one end confrontingsaid seat member and slidably disposed in said cylindrical passageway, aSpring member having a preselected compression strength and disposed insaid barrel member for urging said stem from said valve seat member,pressure cylinder means interconnected to stopper the other end of saidbarrel member and having a cylindrical borehole opening at one endtoward said valve stem and a gas port communicating with saidcylindrical borehole adjacent its other end, means for establishing apressure differential at a location upstream of said valve seat memberand for communicating the upstream pressure at said location to said gasport, and piston means slidably disposed in said cylindrical boreholeand adapted to be urged against said valve stem in response to gaspressure received from said gas port for urging said valve stem intoshutoff engagement with said valve seat member.
 9. The valve assemblydescribed in claim 8, wherein said valve stem is further provided with aspring-engaging portion, and wherein said spring member comprises ahelical spring disposed coaxially about said valve stem andinterconnected with said spring-engaging portion thereof.
 10. The valveassembly described in claim 9, wherein said barrel member is threadedlyinterconnected generally transversely of said body member and is furtherprovided with an internal shoulder portion for supporting andmaintaining said helical spring in a compressed condition, and whereinsaid spring-engaging portion of said valve stem is a flangelike portionlocated at the opposite end of said valve stem from said shutoff surfaceand adapted for abutting engagement.
 11. A blocking valve assemblyresponsive to an excessive pressure differential for interrupting fluidflow, comprising: a body member adapted for interconnection in a flowline, an annular valve seat member mounted in said body member, a hollowbarrel member interconnected with said body member and having acylindrical passageway in one end adjacent to and axially aligned withsaid seat member, a cylindrical valve stem having a shutoff surface atone end confronting said seat member and slidably disposed in saidcylindrical passageway to be urged from said seat member by gas pressurein said body member. a spring member having a preselected compressionspring and disposed in said barrel member for urging said stem from saidvalve seat member. pressure cylinder means interconnected to stopper theother end of said barrel member and having a cylindrical boreholeopening at one end toward said valve stem and at least two gas portseach communicating with said cylindrical borehole adjacent its otherend, piston means slidably disposed in said cylindrical borehole andadapted to be urged against said valve stem in response to gas pressurereceived from one of said gas ports for urging said valve stem intoshutoff engagement with said valve seat member and closure meansstoppering the other of said gas ports and openable for releasing gaspressure from said piston means.